The added words “and not for ours only, but also for the sins of the whole world” point out the true scope of Christ’s atoning sacrifice. … Christ’s atoning work has made it possible for any one in the world to be saved, but each sinner must personally accept Christ as his Redeemer (cf. 1 Tim. 4:10) “Men may—yea, and do—reject the propitiation when they reject the Propitiator—the Lord Jesus Christ.”

D. Edmond Hiebert, The Epistles of John, (Greenville, SC: Bob Jones University Press, 1991), 75-76. He quotes W. E. Vine’s Expository Dictionary in the last sentence.